US2420232A - Signaling system - Google Patents

Description

:May 6, 1947. H B, DEAL 2,420,232

SIGNALING SYSTEM Filed Dec. 21, 1942 2 Sheets-Sheet 1 /ZarfM/ ATTORNEY amm NWN

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illlllllllll .May 6, 1947. H. B. DEAL 2,420,232

SIGNALING SYSTEM Filed Dec. 21, 1942 2 sheets-sheet 2 INVENTOR #mwa/v 045/12.

ATTORNEY Patented May 6, 1947'` UNITED 12STATE S PATENT OFFICE 2,420,232 STGNALING SYSTEM Harmon B. Deal, Glen Ridge, N. J.,

assignor to Radio Corporation of America, acorporation of Delaware Application December 21, 1942, Serial No. 469,604

1 claim. l

` to provide a system of transmitting various color signals, representative of different degrees of probability of air raids, wherein there is utilized a radio frequency carrier for transmitting such color-representative signals.

In the past .there has been proposed various systems for controlling remote devices, whether electrical or mechanical, by imposing upon a radio frequency carrier at least one modulation frequency which functions, upon being received at a remote point, to effect a control function. It may, therefore, be stated that it is an important object of my present invention to provide a radio system for transmitting one or more modulation control frequencies, and the transmitted modulated carrier energy being received by a normal broadcast receiver which has associated therewith selective devices which are responsive to respective control frequencies to effect various control functions.

Another obiect of this invention is to provide a plurality of highly selective reeds responsive to a common source of modulation frequencies, each of which frequencies corresponds to the frequency of a respective reed, and each reed being electrically associated with a respectivev circuit for effecting a control function.

Yet another object of my invention is to provide inv conjunction with a reed driving source of control frequencies, a limiter device which functions to protect the reed-actuating device from extraneous and undesirable noise impulses.

Another object of the invention is to provide in combination with the audio output stage of a radio receiver of the broadcast type, means responsive to predetermined control frequencies for indicating various degrees of probability of an air raid, and there being utilized means for automatically rendering the output stage ineffective.

Another object of my invention is to provide a radio receiver capablel of receiving modulated carrier waves which include modulation control components which may be audible or superaudible; resonant means, whether mechanical or electrical, being .employed for securing response to the components, and the response being utilized selectively to actuate relays controlling work: circuits.

More specific objects of the invention are to improve generally the eiiiciency of modulation control signalling systems, and to provide such a system in a reliable and economical manner.

Other features of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically in Fig.

1 a circuit organization whereby my invention may be carried into effect. Figs. 2 and 3 show-respectively diiferent embodiments of mechanical interlocks for .the relays of the system shown in Fig. 1.

Referring, now, to the accompanying drawing, the audio output stage of a radio broadcast re.. ceiver isv shown in detail. Associated with the audio output stage there are shown the various networks which comprise the invention. Since the nature of the broadcast receiver is immaterial insofar as the functioning of the various control frequency circuits is concerned, it is not necessary to show the circuits which precede the output tube i. The latter may be an audio ampliiier tube of the 35L6GT type. This is a screen grid amplifier tube which has a cathode 2 of the indirectly heated type, a control grid 3, a. plate I and a screen grid between the control grid 3 and plate 4. The control grid 3 is connected through the coupling condenser 5 to the source of audio signals. 'I'he grid is connected to ground through a grid return resistor 6. The input terminal of condenser 5 is connected to ground through a condenser l.'

The source of audio signals may be of any desired form. It may be the output load of the demoduiator of a broadcast receiver, or it may be the plate circuit of one or more prior audio amplifier tubes. The broadcast receiver itself can be of any well known form, such as a superheterodyne receiver operating in any assigned frequency band. For example, the frequency band can be in the 500 to 1750 kilocycle (kc.) band which is employed in amplitude modulation broadcasting. Of course, the receiver can be of the frequency modulation type. In that case the receiver might operate inthe 42-50 megacycle range. Whatever the nature of thetransmission employed it is desired that at the transmitter there be applied one or more control modulation frequencies to the carrier. In the case of amplitude modulation the modulation frequency would be applied to the carrier as an amplitude modulation component, and the modulation component -would appear inthe output circuit of the demodulator at the receiver. Even in the case of frequency modulation the control frequency deviation component at the transmitter would appear at the receiver in the form of a. control audio signal in the demodulator output circuit.

It is to be understood that this application is not limited orrestricted to specic frequency ranges of the control modulation components. These components may be of audible or superaudible frequency value. High audible frequencies may be employed, and in such case electrically resonant; circuits may be substituted for the reeds.

The range of control frequencies may cover up to 100 cycles. In the present system the frequencies of cycles, 30 cycles, 36 cycles and 24 cycles have been chosen by way of illustration. However. the invention is not limited to these specic frequencies. According to the present invention, and again by way of illustration, a 20 cycle signal corresponds to red indication; a 30 cycle signal corresponds to a .blue indication; a 36 cycle signal corresponds to a yellow indication; and the 24 cycle signal corresponds to a.

white indication. Briefly. the white indication may correspond to an All clear signal; the yellow indication may signify Raid possible signal; the blue indication designates Raid probable signal; while the red indication corresponds to Raid certain and imminent signal. 'Ihese colors are purely illustrative, both in significance and use. The designations given are those oi' the Interceptor Command Alert System Ain which four degrees of probability of air raids are indicated.

According to my invention, these various color signals can be turned on and oir by energy transmitted on the carrier of a radio transmitter thereby duplicating in every Way by radio the type of warnings that have heretofore been restricted to Voice warnings over telephone lines. This -is readily accomplished at the radio transmitter by applying to the carrier, with or without audio program existing thereon, the various control frequencies. The manner of applying the modulation frequencies to the carrier is not material. Merely by way oi. illustration, reference is made to my copending application Serial No. 462,252, filed October 16, 1942, now U. S. Letters Patent 2,378,540, granted June 19, 1945, for a method of generating modulation control frequencies at a transmitter.

power supply network. The commercial alter- Assuming, now, that the modulated carrier y energy has been received at the receiver, the same is demodulated so that the modulation signals are transmitted through the audio ampliiier to the signal grid -3 of audio amplier tube I'. The control modulation energy may be separated from the audible components by providing a low pass lter 8 whose input terminals are connected :by lead 9 to the signal grid 3 of audio tube I. If desired the filter 8 could be omitted,' and reliance placed on the R-C networks 4 I-42 and 41-48 located in the limiter input and output respectively. The output terminals of the low pass filter 8 are connected to the input grid of the limiter section of the tube I0. The latter is a twin triode which may be of the 12AH7G2 type. The low pass filter 8 is constructed to pass frequencies up to 100 cycles, and to reject all signal energy above 100 cycles.

In that way the system following the low pass filter will be responsive solely to the control modulation frequencies.

' 'I'he audible signal energy is reproduced in the usual manner by a loud speaker whose voice coll is arranged in circuit with the secondary windingnatingcurrent source. say 60 cycles, is rectified to provide the direct current voltage which is needed for energization of the various electrodes of the tubes. Direct current mains may be used, if desired. The shunt condensers I2 are connected across the rectifier output terminals to provide suflicient filtering to .prevent any pulsating components from passing through to the tubes I and I0. Low frequencies, as from the 20 cycle signal, require high bypass condensers in the power supply network to preclude the plate current changing the power supply voltage when the plate current swings are of fairly high current and low frequency. The high potential, or positive, side ofthe power supply line is indicated by the usual plus sign, whereas the negative side of the line is shown as being at ground potential.

The primary windingl of transformer II has one end thereof connected to plate 4 of tube I, while itsv opposite end is connected through resistor I3 to the lead connecting the screen grid of tube I to the high positive point I4 of the power vsupply network. An intermediate point on the primary winding of transformer II is connected directly to the point I4. This is a humbucking arrangement. and can be replaced by conventional energizing connections. 'I'he cathode 2 is connected through resistor I5 and lead I 6 to a push button-controlled switch I1. The switch II is a spring switch whose contact point is connected to ground. In other words, as long as the switch I'l is closed the audio output tube I is operative, since its cathode circuit is completed back to ground. However, lwhen the push button I8 is depressed thereby opening switch I1, then the audio output tube l is rendered inoperative by virtue of the opening of the cathode circuit thereof.

The control modulation energy could be derived, if desired, from the demodulator output circuit directly. Regardless of the point of origin of the control modulation frequency currents, they are separated out from the audible currents by a low pass filter and/or the R-C filter networks in the limiter input and output. The

control currents are utilized in the manner now to be described. It is desired to have a relay of the electromagnetic type actuated in response to reception of a corresponding control modulation frequency. A high degree of selectivity is imparted to the responsive device by employing a highly selective reed to be actuated by a corresponding control frequency current. Vibration of a reed automatically closes a respective electromagnet relay circuit.

Since four control frequencies are employed in the system, four respective reeds 20, 2i, 22 and 23 are provided. 'I'hese reeds are of well known steel spring form, and are respectively indicated as tuned to 20 cycles, 30 cycles, 36 cycles and 24 cycles. The reeds are shown mounted on a common base 24, and the latter may be the horizontal plate of an L-shaped magnetic armature 25. A plate 26 of steel spring provides a vertical r( silient support for the armature 25. The convex extension of the armature is located adjacent the -core of driving electromagnet 21. The springy support '26 may be secured to any desired nonrigidly affixed to metallic clamps 30, it will be seen that each reed is normally at ground potential.

The free end of each reed may have a vertical white tab 3| so that when vibrating an observer may view the tab and be aware of the vibration. It will be understood that the conguration, position and composition of the reed support elements are purely illustrative, and not restrictive.

The reed drive electromagnet 21 has an energizing winding 32 arranged in the plate circuit of a reed drive amplifier. The input, or control, grid of the latter is fed by the plate circuit of the limiter. While both limiter and reed drive amplier are shown as respective sections of the twin triode tube i0, the sections may be in separate tube envelopes. The limiter section consists of a cathode 33, a plate 34 and a control grid 35. Amplitude limiting is provided by connecting plate `34 to point 36 of the power supply network through resistor 31. Point 36 has a potential of about +183 volts so that no matter how large a signal is applied to the input grid no greater than about 18 volts can appear in the limiter output. A signal of 18 volts is just suicient to drive the following section to actuate the reeds under resonant conditions. The cathode 33 is connected to point 40 of the supply network through leads 38 and 39. Limiting for negative swings of the signals is accomplished by the low plate voltage to the first triode section of the tube |0. Grid 35 is in series with resistor 4| and ccndenser 42. The grid end of resistor 4| is connected to cathode lead 38 by condenser 42', and resistor 43 is connected 'from the junction of 4| and 42 to cathode lead 38.

Condenser 42 and resistor 4| provide one half section R-C illter. Condenser 48' and resistor 41 provide a second half section R-C lter. The condensers of these filter sections shunt the input to the grid of each section of tube I0. At higher frequencies the shunting eiect isv increased. Hence, a frequency discrimination eiect is provided. The condensers 42 and 48are direct current blocking condensers, while resistors 43 and 41 are grid leaks. Hence, the limiter stage has ltering before and after thereby to permit solely the subaudible frequencies to pass.

Limiting for positive swings of the audio signals is accomplished by the grid circuit of the limiter section of tube I0. It is caused by grid current hcw when the impulses applied to grid 35 exceed a predetermined amplitude. Point 40 is less positive than point 36, and, therefore, proper operating potentials are applied to the tube. The amplifier, or reed drive, section of the tube |0 consists of cathode 44, plate 45 and the control grid 46. The grid 46 is connected by resistor 41 and direct current blocking condenser 48 to the plate 34. Plate 45 is connected to point 49 on the power supply network through a path comprising lead 50, winding 32 and lead 5|.

Control grid 46 is returned through filter resistor 41 and resistor 41 to lead 39, and cathode 44 is similarly connected to lead 39. The space current of the amplifier section of tube l0, therefore, flows through winding 32 ofthe reed drive electromagnet. By virtue of the limiter stage extraneous noise impulses will not possess sulcient amplitude to build up large swings of the sharply tuned reed. The tube l0 may be said to function as a type of noise gate to prevent accidental setting oil of a relay by undesired impulses. Concurrently, by virtue of RHC filter sections in the input and output of the limiter section, the limiter acts to pass solely the modulation control fre- 75 6 quencies. Ii desired, the filter s maybe omitted.

When current ilows through winding 32 the result will be to cause vibration of armature 25 and the associated reeds 20 to 23. However, normally the reeds will not have an independent wide amplitude of vibration. 1f the current flowing through winding 32 is of the control frequency of one of reeds 20 to 23, then that one reed will swing in a wider arc than the other reeds. In this way each reed provides a high degree of selectivity for each respective control signal.

An electromagnet relay is provided for each respective reed. Thus` electromagnet has a winding 6l; one end of the winding is connected by a conductor designated Red to a xed contact point 62. The reed 20 is provided with ,a contact point i 2 which contacts point 62 when reed 20 vibrates widely. Slight movement of reed 20 will not result in contact between l62 and 62'. Conductor 63 connects the opposite end of winding 6| to positive potential point 64.v Condenser 65 shunts the winding 6|. Each of reeds 2|, 22 and 23 has a similar electromagnet'electrically associated therewith. Hence, similar numerals, but successively higher by decades, will be used to indicate the corresponding constructional elements.

The function of each of shunt ccndensers 65, 15, and 95 will be described by specific reference to condenser 65. The contacts 62 and 62' touch only for a small part of a second. The inductance of the relay winding precludes any very rapid rise in current in the winding in this short time interval. A condenser with no series re sistance charges to the impressed voltage instantly. The charging current may be of the order of a hundred amperes, Once the condenser 65 is charged it will discharge through the relay winding even if the contacts 62 and 62 never touched again. Contact between 62 andy 62 for merely long enough to charge condenser 65 will insure the relay operation. Resistor 28 limits the charging current through the shunt relay condensers to a, safe value for the reed contacts.

` Without resistor 28 the instantaneous current may reach 100 amperes, and might weld the reed contacts at the instant of make i The electromagnet 60 has associated therewith a pivoted magnetic armature 66 which is normally spring-biased out of contact with a pair of contact points 61 and 88. Similarly, the electromagnet 10 is provided with a pivoted armature 16 which is spring-biased vout of contact with contacts 11 and 18. The electromagnet 80 is provided with an armature 86 which is spring-biased out of contact with contact points 81 and 88. The electromagnet 90 is provided with a pivoted armature 96 which is normally pulled out of contact with contact points 91 and 98.

The armatures 66, 16 and 86 are all connected by a common conductor |00 to one side of the alternating current source of the system. It will be understood that the last-named source is the 60 cycle source which supplies the rectier of the power supply network which has already been described. The armature 96 is connected through switch |0| to the common conductor |00. The return line for the alternating current source is through ground. Each of contacts 61, 11 and 01 is connected through a respective visual indicator to ground. By way of example, each of the indicators may be an incandescent lamp. The lamp |02, which is connected to contact 61, may be a red lamp, the lamp |03 may be a blue lamp, while lamp |04 is shown as a yellow lamp. In the case of contacts 91 and 98, it is contact 90 vswitch I1 thereby opening the which is connected to ground through a white lamp |00. Y a

It will now be seen that the electromagnets 00, 10, 8l and 90 are selectively energized, and

each one of them acts to pull its respective armature against the respective contacts. Concurrently, there occurs simultaneous release oi.' all other armatures due to a mechanical interlock (not shown). There will be a ilow oi alternating current through each respective lamp |02 to |05. Clearly, then, reception o f a particular control modulation frequency is automatically translated into a visual indication in the form of a respectively illuminated lamp of a predetermined color. Assuming that the reed 20 is selectively vibrated in response to the reception of a 20 cycle signal, contact points 02 and 62' will close thereby causing direct current from the power supply network to ilow through Winding 6|. This will energize the electromagnet 50, and cause. it to pull the amature 06 toward contact point 61. As a result the red lamp |02 will be energized, and indicate the air raid warning associated with illumination of that lamp. The same explanation is applicable to each of the other reeds and associated electromagnetic relays.

Concurrently with the visual indicationv described above. there is provided an audible signal. For'example, the audible signal may be a bell 200 whose terminals are connected from ground to energizing line through the circuits now to be described. The ungrounded terminal of the bell is connected to an intermediate point of resistor 20| connected between line 202 andground.

Resistor 20| is a potential divider resistor which permits the bell to operate on the power supply. The receiver is an A. C.D. C. type, and the use of a bell-ringing transformer to ring the bell would preclude its operation on D. C. Assuming a voltage of about 110 volts across resistor 20|, the bell tap thereon would be so chosen that about 20 volts would be applied thereto. If an ordinary series resistor were used the bell contacts burn up, because upon the bell armature opening the contacts the full 110 volts appear thereacross. With divider 20|, however. solely the 20 volts can appear across the bell contacts as a maximum. Between each of contacts 68, 18 and 80 and line 202 there is connected a respective push button-controlled spring'switch. Thus.

spring switch 203 is opened ,upon depression of push button 200. The button 204 is, of course, composed of an insulation material. Hence, depression of the push button 204 spreads the ilared projections of the switch elements, and opens the circuit from contact 68 to line 202.

Similarly. push button 206 controls spring switch 205, and push button 208 controls spring switch 201. In the case of push button I8 and the controlled spring switch l1, the connections are somewhat diiferent. As explained previously, depression oi push button |8 causes opening of cathode circuit of the audio output tube. Simultaneously. by virtue oi the concurrent opening of spring switch |0|, the electrical circuit'from line |00 to the bell Y than an amount that is broken. This is accomplished by connecting line |00 to armature 06 through switch I0 Contact 91 and armature 96 function to complete the circuit to resistor 20|. An insulation bar 300,

rigidly coupling mobile elements of switches |1 one push lbutton releases all others. It would needlessly complicate the drawing to show auch mechanical interlock, as those skilled in the art or push button tuning for receivers are fully acquainted with the speciilc manner of constructing the device. In the same manner. it is desir'- able to have the four relays 00, 10, 00 and 00 mechanically interlocked so that actuation oi any one 'armature at any one time releases all other armatures.

Fig. 2 shows a simple iorm of mechanical interlock for the relays 60, 10, 00 and 00. A simple latch and bar `arrangement is employed for the four armatures 66, 10, 06 and 86. Since relays and 90 arel duplicates or the relays 00 and 10, only the construction of the latter is shown. The connections to the relays and armatures are numbered as in Fig. 1. The latches 000 and 00| are respectively associated with armatures II and 1B. The latching bar is common to all the latches. The latter are normally spring-biased by spring 503. The electromagnets are mounted upon insulation supports 504.. Each armature has the pivoted end, thereof normally springbiased to the position shown at armature 10. A spring 505 bia/ses the armatures.

The latching device of Fig. 2 has the disadvantage that the lips of the latches, and the edge of the armatures that are latched down by these lips, must be very closely aligned. Otherwise there will be a condition where the lip of relay latch 500, for example, hangs onto the armature Il,

when relay 10 is actuated, whereas latch 500.

should release under this condition. An lmprovement in the action, and one that permits very wide mechanical tolerance, is shown in Fig. 3. In this case the eifect of each armature roller 600 is to push the associated latch back further will lust release the latch example) when the relay is because the latching bar radius of the roller further will cause the unactuated (releasing) armature to stay locked when it should release. A second feature of the roller type latch is that slight overtravel of any relay downward releases all relays except the actuated one. Y 1

This is useful in that the assembly of Fig. 2 has the characteristic that ii due to some cause (probably not of any frequent, or likely, occurrence) all relays were locked down due to violent mechanical jarring, or all relays were simultaneously actuated by an overwhelming signal, etc., in-such case the whole system is locked up. and can only be restoredby outside mechanical interference, l. e., electrical impulses could not release the latch on any of the relays. This is not true of the arrangement of Fig. 3 where the sloping shoulder and the overtravel ci the armature arms operates electrically to push the latch bar back enough to release all but the electricallyoperated overtraveled relay. The roller may, of course, be on the armature, or on the latch bar.

It will be understood that the number of latches employed on the latching -bar 500 (to use the above 10 is actuated. 'I'his 502 must go back the than an amount that will correspond to the number of armatures utilized. The invenplurality .of mechan one relay has the effect of releasing all the others. As previously stated. the mechanical interlock for the push buttons is not specically disclosed, because such interlock arrangements are well known in the push button tuning of receivers. See,` for example, U. S. Patents 2,026,980 and 2,213,845, showing push button tuning for radio receivers, wherein actuation of a single button automatically releases the remaining buttons.

A work circuit outlet 300 may be provided so that controlled work circuits can be connected to any one of the respective relays 60, or 80. Thus, outlet 300 has one terminal grounded, while the opposite terminal may be connected to switches 30|, 302 and 303. Closure of any of these three switches results in energization of outlet 300 upon actuation of the respectively associated relay armature. For example, if switch 303 were closed, then reception of a yellow flash energizes outlet 300. An auxiliary bell, outdoor siren, extension light or buzzer could be' plugged into outlet 300. If switches 30! or 302 were alternatively connected, then outlet 300 would be energized by a red signal or blue signal respectively.

The functioning of the system will now be described. It will first be understood that the bell rings every time a specific modulation control frequency is received. The push buttons function to turn oi the bell for each lamp that is lighted. Four degrees of probability of air raids can be indicated by corresponding color signals. These lamp iiashes, or signals, are transmitted by contro1 frequencies. There need be no sequence of lamp lighting. That is, the red lamp can be lighted rst, or following any of the other lights. However, the red lamp having been lighted, the yellow, blue or white signals can be lighted.

The Army alert" (via telephone) has the feature that any change of lamp color from yellow to red or to white is accompanied by a ringing of the telephone bell to call attention to the fact that a new or different lamp has been lighted. This merely precludes the necessity of having an operator watching the lamps at all times. a bell warning has been built into this four chan- -nel alert system. The same relay that lights the new, or different, lamp also turns on the calling bell. Now, the listener may let the bell continue to ring if he so chooses, but usually it is preferred to turn it off. This can be done by pushing the button associated with the lamp that has just ilashed on an acknowledgment.

Pushing the button does two things. It breaks the connection (through the thenactuated relay) to the bell, and simultaneously through the interlocking push button switch, sets up the circuit so that any other relay (i. e., lamp) will, when actuated, cause the bell to ring again. Thus any change of lamp signals will be accompanied by a new outburst of the bell that is stopped by pushing the associated button. If the broadcast station should go back to the lamp color that was showing prior to any change accompanied by the bell ringing, the bell would cease ringing without any attention on the part of the listening operator. For example, suppose the set was in the .All

If, now, the station shouldl clear" condition. flash a yellow signal, the set would turn on, the yellow lamp would light and the bell would ring. If, now, the yellow button is pushed, the bell stops ringing. If it is not pressed acknowledged and the station should then flash the All clear," the set stops, the bell stops, and the yellow light goes off.

Such

, 10 The white, or All clear flash, is the release of the receiver operator. In this case pressing the button silences the set (cathode lead open). silences the bell, and shuts off the white lamp. The opening of the cathode lead, in addition to quieting the set, saves power, and by permitting a slight rise in +B voltageimakes the stand by condition slightly more sensitive. The white lamp is wired a little differently from the other lamps so Vthat when the white button is pressed the white lamp goes oi, as well as turning oft the bell and the set. It could easily be rewired by eliminating switch IOI so that it operated exactly as the other lights do (i. e., stays on when its associated button is pushed), but the All clear lamp need not burn when the set is off and in the "stand by position. White will show when the transmitting changes from some other color to white, and will continue until the A11 clear button is depressed. The push buttons serve an incidental function in that should the bell start to ring, the receiver 0D- erator may note which button is depressed as he approaches the set. If, for example, the blue button had been depressed, and the lamp ashes yellow, he may know from the buttons that the danger is abating.

It is, again, emphasized that the bell 200 will ring every time any lamp is lighted, and will continue to ring until the colored button associated with that lamp is depressed. If, now, a new lamp is lighted, the bell rings again until its button is depressed. The push buttons are mechanically locked, and remain depressed until the pressing of another button on the gang releases the first, and locks the last depressed (like a push-button radio receiving set).

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

In a signalling system, a plurality of differently colored lamps representative of dierent signals, means providing said diilerent signals, a plurality of reeds each selectively resonant to a respective one of said signals, means responsive to said signals for selectively vibrating said reeds, a iplurality of separate lamp energizing circuits each including a respective one of said lamps in circuit, a respective relay-actuated circuit closer included in each of said energizing circuits, each of said reeds being constructed to function as a circuit closer for a respective one of said relays upon reed vibrations, said relays being interlocked so that energization of any one releases allv the others, an alarm bell, a bell energizing circuit respective to each relay-actuated circuit closer, and a plurality of interlocked bell silencing buttons for selectively breaking the respective bell energizing circuits.

HARMON B. DEAL.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS (Other references on following page) Number Number Number Name Date Denton Jan. 12, 1943 Edwards June 20, 1939 FCREIGN PATENTE Country Date German 1 Nov. 25, 1909 British IAUC. 2, 1917 OTHER REFERENCES An article entitled Air Raid Alarm, appearing onpazee 15, 17 and 35 of "Radio," June 1942.

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